1. Field of the Invention
The present invention relates to an exhaust device in a combustion engine for controlling an exhaust gas flow in an exhaust passage to enhance exhaust efficiency etc., and a motorcycle equipped with the exhaust device.
2. Description of the Prior Art
Generally, as shown in FIG. 11, an exhaust device in the combustion engine has a plate-shaped valve body (butterfly valve) 102 arranged in an exhaust passage 101 in an exhaust pipe unit 100. The valve body 102 is fixed to a valve shaft 103. Both ends of the valve shaft 103 are rotatably supported by left and right bearing members 105, 105. Exhaust gas flow is controlled by changing a rotation or turn angle (opening angle) of the valve body 102 according to an operation state of the combustion engine thereby enhancing the exhaust efficiency. When the engine is in high-speed rotation, output efficiency of the engine is enhanced by positioning the valve body 102 in a full-opened state as shown with a solid line. When the engine is in low-speed rotation or idling, generation of noise and the like is suppressed by positioning the valve body 102 in a substantially full-closed state as shown with a virtual line.
During operation of the combustion engine, temperature inside exhaust passage 101 becomes high due to heat of the exhaust gas, and thermal deformation occurs at the valve body 102 and the valve shaft 103. In this case, since the conventional valve shaft 103 is assembled to bearing members 105, 105 with a play (gap) of a predetermined clearance C3 in an axial direction of the valve shaft 103, it is possible to avoid a decrease in the smoothness in the operation of the valve body 102 by the thermal deformation.
In other words, stopper faces 108a, 108a facing end faces 105a, 105a on the inward side of the exhaust passage of each bearing member 105, 105 in the axial direction are arranged on both ends in the axial direction of the valve shaft 103, so that an interval L2 in the axial direction of the stopper faces 108a, 108a is shorter than an interval L1 in the axial direction of the end faces 105a, 105a of the bearing members 105, 105 by the predetermined interval C3 (e.g., about 1 mm to 2 mm).
The valve shaft 103 is biased to one side, such as the left side, in the axial direction by one coil spring 110, and at the same time, biased towards the valve opening side about an axis O1 of the valve shaft. That is, when the valve shaft 103 is biased towards the left side by the load in the axial direction of the coil spring 110, the left stopper face 108a comes into contact with the end face 105a of the left bearing member 105, and at the same time, the valve shaft 103 is biased towards the valve opening side by the restoration force in the torsional direction of the coil spring 110. The set load value in the axial direction by the coil spring 110 is about 5N (about 0.5 kgf) etc., and the set torsional torque value is about 350N·mm (35 kg·mm) etc. The prior art document includes Japanese Laid-Open Utility Model Publication No. H2-101035.
In a structure in which the valve shaft 103 is assembled with a play (clearance C3) in the axial direction as in FIG. 11, smoothness in the operation of the valve body 102 is maintained even if the valve shaft 103 etc., is thermally deformed. However, when dynamic pressure of the exhaust gas is applied on the valve body 102 in the exhaust gas flow direction and the vibration of the combustion engine is transmitted to the valve body 102 during the operation, the valve shaft 103 and the valve body 102 rattle thereby producing “rattling” noise, which might bring discomfort to the passenger. In particular, when closing the valve body 102 from the full-opened state shown with the solid line to the substantially full-closed state shown with the virtual line, the dynamic pressure of the exhaust gas acting on the valve body 102 becomes larger as the substantially full-closed state is approached. As a result, the valve body vibrates drastically near the substantially full-closed state, and the noise is likely to be produced.
In the case that a bushing made of metal is used as a bearing member 105, 105, an abrasion sound (metal sound) of “screeching” sound is produced between the metals when the valve body 102 is rotated if solid components in the exhaust gas such as carbon enters in the fitting surface of the bearing member 105 and the valve shaft 103, which might bring discomfort to the passenger.